Silicon single crystals, which are used as substrates of semiconductor integrated circuits, are typically manufactured by the Czochralski (CZ) method or the magnetic field applied Czochralski (MCZ) method. The CZ and MCZ methods involve charging a silicon raw material to a quartz crucible, melting the material into a melt, bringing a seed crystal into contact with the melt, and pulling the seed crystal to grow a silicon single crystal. In a single-crystal manufacturing apparatus (a pulling apparatus) according to the CZ or MCZ method, a heater to heat the melt is disposed in a main chamber, and the quartz crucible to receive the melt is disposed inside the heater.
The methods normally begin with charging of the raw material to the quartz crucible. The raw material is then melted by the heater. In recent years, enlarging the diameter and length of silicon single crystals may cause lack of the raw material initially charged to the quartz crucible, creating a need for adding the raw material. This is called additional charge. The additional charge is carried out by storing additional raw material in a recharge tube including a cone (a conical valve) at its lower end, and feeding the raw material into the quartz crucible from the recharge tube. After all the raw material is melted, growth of a silicon single crystal begins.
The quartz crucible is filled with the melt, into which the raw material is melted. The silicon single crystal is grown from the melt. The grown single crystal is pulled into a pull chamber that is connected to the upper part of the main chamber through a gate valve, and cooled there. The single crystal is then taken out of the pull chamber.
In single-crystal manufacture, if one single crystal is manufactured with a single quartz crucible, the growth of the single crystal is completed at this point of time. In this case, however, the production cost increases because the quartz crucible is broken at the end of the growth and therefore cannot be reused. A multiple operation that a plurality of single crystals grow by using a single quartz crucible may accordingly be carried out. In this operation, growing a single crystal reduces the melt in the quartz crucible by the amount corresponding to the grown crystal, thereby making it impossible to grow a next single crystal. The raw material is accordingly recharged to compensate for the reduction.
There have been proposed recharging methods such as a rod recharging method, and a method of recharging raw material from a container, as disclosed in Patent Document 1. The art disclosed in many patent documents involves storing the raw material in the recharge tube including the conical valve at its lower end, and feeding the raw material stored in the recharge tube into the quartz crucible. Patent Documents 2 and 3 disclose the foundation of this art.
Such a method feeds the raw material as follows: after a grown silicon crystal is taken out of the pull chamber with the gate valve closed, the recharge tube storing raw material is hanged by a wire and fixed. Air is evacuated from the pull chamber so that the pressure in the pull chamber is the same as in the main chamber. The gate valve is then opened to lower the recharge tube. The conical valve is lowered to open the opening of the recharge tube and thereby to feed the raw material.
Since broken chunks of polycrystalline silicon, although single crystal is rarely used, are normally used as the raw material to be charged, there are vacant spaces in the recharge tube storing the material. A single charging process by the recharge tube may accordingly be insufficient for additional raw material to grow a single crystal. In this case, the charging process is successively performed multiple times.